[dataimport, migration] more fixes in the spirit of a6c32edabc8d:
[entity, metadata] huuum, use resolvable url as cwuri... And fix existing ones.
Closes #3390388
change_attribute_type('CWCache', 'timestamp', 'TZDatetime')
change_attribute_type('CWUser', 'last_login_time', 'TZDatetime')
change_attribute_type('CWSource', 'latest_retrieval', 'TZDatetime')
drop_attribute('CWSource', 'synchronizing')
add_attribute('CWSource', 'in_synchronization')